ST. LOUIS The worst injury the Steelers could imagine occurred Sunday and they received good news and bad news after Ben Roethlisberger had an MRI Sunday night in Pittsburgh.

Roethlisberger, the player who most defines their chances at another championship, departed the game Sunday in the third quarter with a left knee injury. An MRI showed he has a sprained MCL and a bone bruise, which means he should return this season but likely will miss 4 to 6 weeks, according to a source close to the situation.

The Steelers fate now rests in the hands of backup quarterback Mike Vick, halfback Le'Veon Bell, wide receiver Antonio Brown and a defense that for the second week in a row showed surprising spunk.

"It's always disheartening when your leader takes a fall and goes down," said Brown, the NFL's leading receiver who added another 108 yards on 11 receptions. "I've never seen him down so it was a little shocking, so I just asked him how he was doing. He said he was all right and he'll be back."

Yet Brown, Vick and the rest will have their work cut out for them at least Thursday night at Heinz Field against the Baltimore Ravens, even though their brutal rivals are 0-3.

Maybe, just maybe, their defense that has played above most everyone's expectations the past two games can go one better.

"I just think we have to step it up a notch, step it up maybe a few notches," said safety Will Allen, whose interception of a Nick Foles pass late in the game sealed the victory. "Baltimore, they're a rugged opponent. We have to help out our offense more by giving them the ball more."

The Steelers made it through the two-game suspension of Bell, have survived the losses of All-Pro center Maurkice Pouncey to injury that will keep him out at least half the season and No. 3 wide receiver Martavis Bryant to a four-game suspension. Now they must somehow find a way without Roethlisberger, who at 33 was at his peak performance in what might be a Hall of Fame career.

"Ben's injury is a tough one because he's our starting quarterback of this team," defensive end Cam Heyward said. "But it's time for other guys to step up. We're not going to shy away from any competition."

They did not do that Sunday and, in fact, provided the kind of defense that used to be the norm for them. Roethlisberger directed scoring drives on each of his first two series but the offense came up empty after that. He left with a 9-3 lead.

This was a game the defense won. It held the Rams to 71 yards rushing, held Foles to 197 yards passing, sacked him twice, and Will Allen intercepted a deep pass with 2:51 left that set up Josh Scobee's second field goal to virtually seal the win with 1:56 left.

It was the Steelers' first interception this season.

"We're satisfied with not giving up big plays, we're satisfied with teams not scoring touchdowns," Allen said. "It helps just to win. When we have a chance for interceptions we'll get 'em. We have to take advantage of them and this time we did."

That defense held the Rams to a second Greg Zuerlein field goal with 5:38 left after St. Louis had reached a first down at the 7.

"We really had to bear down," Heyward said of a Rams drive that started from their 29.

"We understand they started to get a little momentum, but we just played through it."

Vick held his own, although he nearly lost a fumble early in the fourth quarter that Bell recovered at the Steelers 27. A pass also would have been intercepted except that it was dropped. He completed his other five for 38 yards, including a 7-yarder on third down to Darrius Heyward-Bey to the St. Louis 23 that preceded Scobee's late field goal.

"I wish it could have been better," said Vick, signed Aug. 25 after backup Bruce Gradkowski went on injured reserve. "I wish I could have done some things differently, but that will come with weeks of work. ... I was brought here for a reason, so I have to hold it down until No. 7 gets back."

They all will.

"He's a guy who's been in this league for a long time and won a lot of games," Bell said of Vick. "He played in championships, so I'm very confident. Once we plug him in, we just need to keep it going."

Roethlisberger was injured on what appeared to be a hit against the rules. St. Louis safety Mark Barron crawled on the turf and then dived into the Roethlisberger's lower legs for a sack.

According to the NFL rules on protecting the passer, "No defensive player who has an unrestricted path to the quarterback may hit him flagrantly in the area of the knee(s) or below when approaching in any direction."

No penalty was called and Steelers coach Mike Tomlin said he had no issue with the hit.

"I stumbled on the way in and just caught his leg," Barron said. "I caught him down around his lower leg. He was grabbing his knee. I didn't have anything to do with his knee."

The Steelers left St. Louis knowing they would be without their quarterback for the time being.